I got your email, Louise - Sorry, I've been away at a bonsai show & visiting friends/family. This is a tough one. As I understand it, you have a concrete block planter constructed atop a concrete slab, with no way for the water to make its way into the sand below the slab. Drilling holes in the block walls won't improve drainage. The only 2 ways to effectively improve drainage would be to make sure there is a 'bridge' between the soil in the planter and the sand below the concrete so the water can percolate through the soil, or to make sure the soil you use is porous enough that it holds little or no perched water. This would mean that your soil particles would have to be large & stable, insofar as their structure is concerned. Even the soil you see me holding above would be inappropriate for your application because that soil depends on the wicking action of the earth (in RBs) to ensure it doesn't remain saturated after rain/watering. It looks great, but if you have high standards it would be a disaster in a container or in your app. I'm thinking that about an 80% fraction of a mix of all purpose sand and all purpose gravel and 10% each of pine bark fines and Michigan (reed/sedge) peat or good compost would probably be the direction I would head. In subsequent years, I think all you'll need to do is mulch with pine bark to keep the soil healthy. If expense isn't a major consideration, you could use Turface MVP as the base ingredient for your soil. Topsoil and fine sand just isn't going to drain w/o help. The alternative would be to remove the old soil and cut a hole in the slab so water can drain from the soil in the planter, but it would have to be a hole of considerable size - preferably at the lowest point on the planter's bottom. If you have pictures, it's possible that something else might be devised that will allow the water to move out of the soil via some sort of passive wicking mechanism that we/you could devise, but I'd have to see the application in order to envision the possibilities. There might be a way to employ a false bottom by placing FRP panel on top of cement blocks that are resting on the bottom & building it like a self-watering container in reverse - where you employ a wicking column of soil to REMOVE water instead of supplying it, but I don't know how far you want to chase the issue? ;-) AL

Are you considering using a slab instead of block walls for a foundation? If so, you may need to consider the frost depth in your area in comparing prices. Most places, you need to go deeper with your foundation than the most extreme depth to which the ground will freeze. If you don't do so, your foundation will move with the ground freeze/thaw cycles. Probably not an issue with RR ties (unless you have glass glazing), but the movement will definitely crack block or a slab foundations. And will make the GH structure attached to them move too, which may cause doors not to open, or break brittle glazing, like glass. Most slabs are poured with a thicker perimeter, say 8" wide or so which goes down to the frost line. Usually some rebar is put in as well. There are also potentially other requirement in various areas depending upon your soil types and ground moisture, etc. If you were just doing a block wall foundation, you typically would still need to dig them down below the frost line, and pour a footing for them (a concrete ledge, usually 16" wide and 8" deep with rebar), and then set them on that footing. (I said this poorly, it is the bottom of the footing which needs to be below frost line). Unless you are in a temp zone where ground freezing isn't an issue, you should investigate the frost line depth in your area. And other foundation practices there too. They may not be needed for a simple (and flexible) GH, but always good to know how things are being done in your area, so you can decide how much to do for your project.

Well, The engineer did provide an updated design and stamp of approval on the poured product. The builder moved quick to remedy the situation. I'm still a bit concerned though, as I contacted several structural engineers and 2 of them said they would NEVER add-on as described and the other 2 said it was perfectly acceptable. I'm waiting to hear back from the PTI (Post-Tensioning Institute) to see what they say. I don't know that I have any recourse except to walk away from my deposit which is not an insignificant amount of money. Any yes, the soil conditions and cost both contribute to the use of the post-tensioned slab which is the most common construction method here in Houston.

My mistake Mike, I thought it was 1.5 ft. not inches. I've never seen a slab resting on footings, new one on me. Sounds like you would need to fasten sleeper joist to the slab, then screw 1x6's or 1x4's for decking. Each joist would have to be ripped to compensate for slop of the slab. Leave the ends open & use wider gaps than normal then maybe you won't have condensation problems. With all that said I'd be inclined to demo that slab & start from stratch. Al